Antennas may be provided to effectively radiate radio waves, and also to; receive radio waves. Antennas can be anything from a simple wire, to a complex array of antennas coupled together such as in a radar antenna. However, all antennas share a common goal of either transforming an electrical signal to a radio wave, or transforming a radio wave into an electrical signal for further processing. Whether receiving a radio wave or transmitting one, an antenna will typically be designed to either take in the most signal possible, or transmit the most signal possible.
Antennas may be considered to be one port devices since this is typically one physical connection to an adjoining circuit. A typical aim in antenna design is to match the impedance at the port of the antenna to that of a transmitter, or other desired radio circuit (for example, an antenna switch, duplexer, low pass filter or the like). In a matched condition, a maximum amount of signal or signal power from the transmitter may be coupled to the antenna, and then radiated into the air. When receiving, a good match allows the maximum amount of power in a received radio signal to be coupled to the subsequent receiver circuits, such as a low noise amplifier (“LNA”) or the like. Thus, a good match may be useful for receiving or transmitting a radio signal efficiently.
A match is typically far from perfect across a band of frequencies. Antennas are reactive devices. Thus, they have associated them with capacitance, and/or inductance. Capacitance and inductance are inherent energy storage parameters whose impedance is frequency dependent. That is when the frequency changes the impedance at the antenna port will change and the match at the antenna port will also change. A designer may find that an antenna optimized to provide the best possible match at one frequency, will provide an unsatisfactory match at another frequency in the band of operation. A designer will often seek to design an antenna that has an acceptable match across the band of operation, or even several bands of operation that may not necessarily be the best one that could be obtained at a single frequency.
This is one reason why there are so many types of antennas. Aside from antennas designed to optimize other parameters, such as antenna gain, beam shaping, and the like, different designs may be needed to achieve a good match at the operating frequency. Achieving an acceptable match over a given range of frequencies can be difficult and may require differing configurations to achieve the desired results.
Other parameters that can affect an antenna's design and final form, may include the environment the antenna is used in and the space available for the antenna. The presence of objects near the antenna, such as a cell phone user's head, large metal surfaces and other antennas can also affect the design. Physical space constraints can lead to configurations such as antennas that pull out from the body of a cell phone and antennas that appear to be a knob or stub on the telephone case and other unique designs.
In particular, the presence of other antennas can affect an antenna's performance. For example, an antenna receives energy just as easily as it transmits it. If two antennas are located next to each other and one is transmitting while the other is receiving, the energy from the transmitting antenna can impinge on and interfere with the adjacent antenna's associated receiver. It does not matter that each of the antenna's are optimized for different frequency ranges. If the antennas are close, even a weak coupling (due to different bands of operation) between the antennas will still transmit a substantial amount of energy into the receiver from the adjacent antenna transmitter. Energy from an antenna falls off at roughly the inverse of the square of the distance from the antenna. Thus, when the antennas are close, the density of energy impinging on an adjacent antenna and receiver can be quite strong.